The present invention relates generally to an organic light emitting device (OLED), and more particularly to a thick (100 to 250 nanometers) porphyrin buffer layer between the anode and the luminescent region of an organic light emitting device (OLED).
Organic light emitting devices (OLEDs) represent a promising technology for display applications. A typical organic light emitting device includes a transparent or a substantially transparent substrate; a transparent or substantially transparent first electrode, which usually acts as a hole-injecting anode; a luminescent region comprising an organic electroluminescent material; and a second electrode, which usually acts as an electron injecting cathode.
When a voltage is applied across the first and second electrodes, electrons are injected from the cathode into the luminescent region and holes are injected from the anode into the luminescent region. The holes and electrons recombine in the luminescent region, which emits light through the anode and through the substrate.
To achieve efficient electroluminescence, some known OLEDs have an organic luminescent region consisting of two or three or four extremely thin layers of less than 1 micron in combined thickness separating the anode and cathode. The layers include a hole transport layer (HTL) adjacent to the anode to inject and transport holes and an electron transport layer (ETL) adjacent to the cathode to inject and transport electrons and sometimes also acting as the light emission zone of the OLED. During operation, an applied electric field causes positive charges (holes) and negative charges (electrons) to be respectively injected from the anode and the cathode to recombine in the luminescent region and thereby produce light emission.
The thin luminescent region offers reduced electrical resistance, permitting higher current densities for a given level of electrical biasing. Since light emission is directly related to current density through the luminescent region, the thin layers coupled with increased charge injection and transport efficiencies have allowed acceptable light emission levels (e.g., brightness levels capable of being visually detected in ambient light) to be achieved for the first time with low applied voltages.
Electroluminescence can be obtained from organic light emitting devices containing mixed layers in the luminescent region, for example, layers in which both the hole transport material and the electron transport material are mixed together in one layer.
In general, the luminescent region between the anode and cathode electrodes of the OLED is very thin organic medium, usually less than 200 nm thick. In order to enable a low OLED operating voltage of less 15 Volts, the total thickness of the organic luminescent region, which may comprise one or more layers, typically two to four layers, has to be minimized to less than 1000 nanometers and preferably less than 200 nanometers.
A thin buffer layer of less than 100 nanometers thickness made of a porphyrin compound such as copper phthalocyanine (CuPc) can be formed between the anode and the hole transport layer (HTL) of the OLED to increase the operational stability of the OLED as taught, for example, in U.S. Pat. Nos. 4,356,429, 4,720,432 and 4,769,292, herein incorporated by reference. In these OLEDs with thin porphyrin buffer layers, the hole transport zone (HTZ) comprises one or more layers of a hole transport layer (HTL), each comprising a hole transport material (HTM) in addition to the buffer layer. Although the inclusion of the thin buffer layer in these devices is necessary for achieving a satisfactory operational stability, such as a half-life amounting to several thousands of hours, the thin buffer layer leads to an inevitable and undesirable increase in OLED operating voltage from three to four Volts higher than an OLED without a buffer layer. A total thickness for the whole organic luminescent region, including the thin porphyrin buffer layer, is suggested to not exceed 1000 nanometers, and preferably, to not exceed 200 nanometers. Smaller thicknesses for the thin buffer layer of less than 40 nanometers are used in these OLEDs, where the smaller thickness, as then believed, was desirable in order to avoid further increases in OLED operating voltage.
Another known problem with OLEDs, in general, is the high susceptibility of the devices to electrical shorting. The reduced thickness of the organic luminescent region is necessary to allow low device operating voltages of less than 15 Volts. Unfortunately the same reduced thickness of the organic luminescent region often results in the formation of pinholes. These pinholes act as electrical shorts between the cathode and anode and eliminate luminescence by the OLED. Although non-conductive polymeric binders can be added, usually as solid solvents, to provide pinhole free layers, and to create a luminescent region of the desired thickness, the binders tend to interfere with the injection of holes and electrons at the electrodes.
It is an object of the present invention to provide a porphyrin buffer layer in an OLED.
According to the present invention, an organic light emitting diode (OLED) has a thick (100 to 250 nanometers) porphyrin buffer layer between the anode and the luminescent region. The thick porphyrin buffer layer in the OLED results in a smaller increase in device operating voltage, only one to two Volts over that of OLEDs without the buffer layer. The thick porphyrin buffer layer reduces the possibility of OLED shorting. The thick porphyrin buffer layer can improve color purity in blue emitting OLEDs due to its significant optical absorption in the red range.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.